This invention relates to semiconductor packaging.
Portable electronic products such as mobile phones, mobile computing, and various consumer products require higher semiconductor functionality and performance in a limited footprint and minimal thickness and weight at the lowest cost. This has driven the industry to increase integration on the individual semiconductor chips.
More recently the industry has begun implementing integration on the “z-axis,” that is, by stacking chips, and stacks of up to five chips in one package have been used. This provides a dense chip structure having the footprint of a one-chip package, in the range of 5×5 mm to 40×40 mm, and obtaining thicknesses that have been continuously decreasing from 2.3 mm to 0.5 mm. The cost of a stacked die package is only incrementally higher than the cost of a single die package and the assembly yields are high enough to assure a competitive final cost as compared to packaging the die in individual packages.
The primary practical limitation to the number of chips that can be stacked in a stacked die package is the low final test yield of the stacked-die package. It is inevitable that some of the die in the package will be defective to some extent, and therefore the final package test yield will be the product of the individual die test yields, each of which is always less than 100%. This can be particularly a problem even if only two die are stacked in a package but one of them has low yield because of design complexity or technology.
Another limitation is the low power dissipation of the package. The heat is transmitted from one die to the other and there is no significant dissipation path other than through the solder ball to the motherboard.
A further limitation is electromagnetic interference between the stacked die, particularly between RF and digital die, because there is no electrical shielding of either die.
Another approach to integrating on the “z-axis” is to stack die packages to form a multi-package module. Stacked packages can provide numerous advantages as compared to stacked-die packages.
For instance, each package with its die can be electrically tested, and rejected unless it shows satisfactory performance, before the packages are stacked. As a result the final stacked multi-package module yields are maximized.
More efficient cooling can be provided in stacked packages, by inserting a heat spreader between the packages in the stack as well as at the top of the module.
Package stacking allows electromagnetic shielding of the RF die and avoids interference with other die in the module.
Each die or more than one die can be packaged in a respective package in the stack using the most efficient first level interconnect technology for the chip type and configuration, such as wire bonding or flip chip, to maximize performance and minimize cost.
The z-interconnect between packages in a stacked multi-package module is a critical technology from the standpoint of manufacturability, design flexibility and cost. Z-interconnects that have been proposed include peripheral solder ball connection, and flexible substrate folded over the top of the bottom package. The use of peripheral solder balls for z-interconnects in stacked multi-package modules limits the number of connections that can be made and limits design flexibility, and results in a thicker and higher cost package. Although the use of a flexible folding substrate provides in principle for design flexibility, there is no established manufacturing infrastructure for the folding process. Moreover, the use of a flexible folding substrate requires a two metal layer flex substrate, and these are expensive. Furthermore the folded flexible substrate approach is restricted to low pincount applications because of limits in routing the circuitry in two metal layer substrates.
The various z-interconnect structures are described in further detail with reference to FIGS. 1A, 1B and 2-4.
FIG. 1A is a diagrammatic sketch in a sectional view illustrating the structure of a standard Ball Grid Array (“BGA”) package, well established in the industry, that can be used as a bottom package in a stacked multi-package module (“MPM”). The BGA, shown generally at 10, includes a die 14 attached onto a substrate 12 having at least one metal layer. Any of various substrate types may be used, including for example: a laminate with 2-6 metal layers, or a build up substrate with 4-8 metal layers, or a flexible polyimide tape with 1-2 metal layers, or a ceramic multilayer substrate. The substrate 12 shown by way of example in FIG. 1A has two metal layers 121, 123, each patterned to provide appropriate circuitry and connected by way of vias 122. The die is conventionally attached to a surface of the substrate using an adhesive, typically referred to as the die attach epoxy, shown at 13 in FIG. 1A and, in the configuration in FIG. 1A, the surface of the substrate onto which the die is attached may be referred to as the “upper” surface, and the metal layer on that surface may be referred to as the “upper” metal layer, although the die attach surface need not have any particular orientation in use.
In the BGA of FIG. 1A the die is wire bonded onto wire bond sites on the upper metal layer of the substrate to establish electrical connections. The die 14 and the wire bonds 16 are encapsulated with a molding compound 17 that provides protection from ambient and from mechanical stress to facilitate handling operations, and provides a surface for marking for identification. Solder balls 18 are reflowed onto bonding pads on the lower metal layer of the substrate to provide interconnection to the motherboard (not shown in the FIGS.) of a final product, such as a computer. Solder masks 125, 127 are patterned over the metal layers 121, 123 to expose the underlying metal at bonding sites for electrical connection, for example the wire bond sites and bonding pads for bonding the wire bonds 16 and solder balls 18.
FIG. 1B is a diagrammatic sketch in a sectional view illustrating the structure of a BGA, generally similar to the BGA 11 shown in FIG. 1A, except that here the molding 117 completely covers the substrate as well as the die and wire bonds, The molding configuration of FIG. 1B is formed by applying the molding compound over an array of a number of BGAs, curing the molding, and then separating the encapsulated packages, for example by saw singulation. Typically the molding in such a package has vertical walls at the edges of the package. In such a package, unlike a BGA as in FIG. 1A, no marginal portion of the upper surface of the substrate 12 is exposed and, accordingly, no electrical traces are exposed on the upper surface of the substrate. Many smaller packages currently are saw-singulated packages, often referred to as “chip scale packages.”
FIG. 2 is a diagrammatic sketch in a sectional view illustrating the structure of an example of a 2-stack MPM, generally at 20, in which the z-interconnect is made by way of solder balls. In this MPM a first package (which may be referred to as the “bottom” package) is similar to a standard BGA as shown in FIG. 1A (and similar reference numerals are employed to point to similar features of the bottom package in FIGS. 1A and 2). A second package (which may be referred to as the “top” package) is stacked on the bottom package and is similar in structure to the package of FIG. 1B (and similar reference numerals are employed to point to similar features of the top package in FIGS. 1B and 2), except that the solder balls in the top package are arranged at the periphery of the top package substrate, so that they effect the z-interconnect without interference with the encapsulation of the bottom BGA. Particularly, the top package in FIG. 2 includes a die 24 attached onto a substrate 22 having at least one metal layer. The top package substrate 22 shown by way of example in FIG. 2 has two metal layers 221, 223, each patterned to provide appropriate circuitry and connected by way of vias 222. The die is conventionally attached to a surface of the substrate (the “upper” surface) using an adhesive, typically referred to as the die attach epoxy, shown at 23 in FIG. 2.
In the top package in the MPM of FIG. 2, as in the bottom package, the die is wire bonded onto wire bond sites on the upper metal layer of the substrate to establish electrical connections. The top package die 24 and wire bonds 26 are encapsulated with a top package molding compound 27. Solder balls 28 are reflowed onto bonding pads located on the peripheral margin of the lower metal layer of the top package substrate to provide z-interconnection to the bottom package. Solder masks 225, 227 are patterned over the metal layers 221, 223 to expose the underlying metal at bonding sites for electrical connection, for example the wire bond sites and bonding pads for bonding the wire bonds 26 and solder balls 28.
The z-interconnection in the MPM of FIG. 2 is achieved by reflowing the solder balls 28 attached to peripheral bonding pads on the lower metal layer of the top package substrate onto peripheral bonding pads on the upper metal layer of the bottom BGA. This type of z-interconnect requires that the upper and lower substrates be designed with matching pads for the interconnect balls. If one of the packages is exchanged for one in which the substrate has a different pad arrangement (different size or different design), then the substrate for the other package must be reconfigured accordingly. This leads to increased cost for manufacture of the MPM. In this configuration the distance h between the top and bottom packages must be at least as great as the encapsulation height of the bottom package, which may be 0.25 mm or more, and typically is in a range between 0.5 mm and 1.5 mm. The solder balls 28 must accordingly be of a sufficiently large diameter that when they are reflowed they make good contact with the bonding pads of the bottom BGA; that is, the solder ball 28 diameter must be greater than the encapsulation height. A larger ball diameter dictates a larger ball pitch that in turn limits the number of balls that can be fitted in the available space. Furthermore the peripheral arrangement of the solder balls forces the bottom BGA to be significantly larger than the mold cap of a standard BGA. Furthermore the peripheral arrangement of the solder balls increases the overall package size (the size increases according to the number of ball rows and the ball pitch). In small BGAs, usually referred to as Chip Scale Packages (“CSP”), the package body size is 1.7 mm larger than the die. In standard BGAs the body size can be as much as about 2-3 mm larger than the mold cap. Moreover, the top package in such a configuration must be made of comparable size to the bottom one even though it may contain a small chip with many fewer interconnects. In this configuration the top package substrate must have at least 2 metal layers to facilitate the electrical connections.
FIG. 3 is a diagrammatic sketch in a sectional view illustrating the structure of an example of a known 2-stack flip chip MPM, shown generally at 30. In this configuration the bottom BGA flip chip package includes a substrate 32 having a patterned metal layer 31 onto which the die 34 is connected by flip chip bumps 36, such as solder bumps, gold stud bumps or anisotropically conducting film or paste. The flip chip bumps are affixed to a patterned array of bump pads on the active surface of the die and, as the active surface of the die faces downward in relation to an upward-facing patterned metal layer of the substrate, such an arrangement may be referred to as a “die down” flip chip package. A polymer underfill 33 between die and substrate provides protection from ambient and adds mechanical integrity to the structure. Such a flip chip package, in which the substrate has a metal layer on only the upper surface, is connected to the underlying circuitry (such as a motherboard, not shown in the FIG.) by solder balls 38 connected to the metal layer through solder vias 35.
The top BGA in this configuration is similar to the bottom BGA, except that the top 35 BGA has z-interconnect solder balls 338 connected (through solder vias 335 in the top substrate) to the metal layer 331 only at the periphery of the top substrate. Solder balls 338 are reflowed onto the metal layer 31 of the bottom substrate to provide the z-interconnect.
Particularly, the top BGA in this configuration includes a substrate 332 having a patterned metal layer 331 onto which the top BGA die 334 is connected by flip chip bumps 336. Between the top BGA die and substrate is a polymer underfill 333. A structure as in FIG. 3 is more appropriate for high electrical performance applications, but it has similar limitations to configurations of the type shown in of FIG. 2. It presents an improvement over the FIG. 2 configuration in that the bottom BGA has no molding, allowing for use of smaller diameter (h) solder balls at the periphery of the top BGA for connection between the packages.
Particularly, this structure is more appropriate, for example, for modules containing stacks of identical memory chips having the same connections at the same locations to facilitate the z-interconnect. It is not appropriate for stacking packages that have different chips with connection points not located over one another in the stack. As in the configuration of FIG. 2, the peripheral arrangement of solder balls limits the number of interconnects. Moreover, the packages must necessarily be of comparable size, and where the chip in one package is smaller than that in the other, the package having the smaller chip will be forced to a larger size, imposing a higher cost.
FIG. 4 is a diagrammatic sketch in a sectional view illustrating the structure of an example of a known 2-stack folded flexible substrate MPM, shown generally at 40. The bottom package in the configuration of FIG. 4 has a 2-metal layer flexible substrate onto which the die is bonded via small beams to the first metal layer of the substrate. The second metal layer of the bottom package substrate carries the solder balls for connection to the underlying circuitry, such as a motherboard (not shown). The substrate is large enough to be folded over the top of the package, thus bringing the electrical interconnect lines upward where they are available for connection to the top package (an example of which is described below) by way of an array of solder balls on the top package. The space around the die and between the die and folded-over substrate is encapsulated 47 to provide protection and rigidity.
Referring to FIG. 4, the two-metal layer bottom package substrate 42 includes a first metal layer 141 and a second metal layer 143, each patterned to provide appropriate circuitry and connected by way of vias 142. A part of the first metal layer, over a part of the bottom substrate, is processed (for example, using an array of punches) to present an array of cantilever beams or tabs 46 arranged to correspond to an array of interconnect pads on the active surface of the bottom package die 44. Over this part of the substrate 42, which may be referred to as the “die attach part”, the first metal layer 141 faces upwardly. The die is aligned, active surface downward, over the die attach part of the substrate, and the cantilevers and the corresponding interconnect pads are joined, typically for example by a “thermosonic” process employing a combination of pressure, heat, and ultrasonic energy to complete the electrical connections. The die 44 is affixed using an adhesive 43, typically a die attach epoxy, onto the die attach part of the flexible substrate 42. A second metal layer 143 of the bottom package substrate 42 faces downwardly in the die attach part of the substrate. Solder balls 48 are reflowed onto bonding pads located on an array on the downward-facing part of the second metal layer 143 to provide for interconnection of the MPM to underlying circuitry (not shown). A solder mask 147 is patterned over the second metal layer 143 to expose the underlying metal as bonding sites for electrical connection, including the bond pads for connection with the underlying circuitry by way of solder balls 48, and the bond pads for connection with the top package by way of solder balls 18, as described below.
Another part of the bottom package substrate 42, extending adjacent the die-attach portion, is folded up and over the bottom package die 44. On this folded-over portion of the flexible substrate 42 the first metal layer 143 faces upwardly. In the configuration of FIG. 4 the top package is generally similar to the BGA of FIG. 1, in which the die is wire bonded onto wire bond sites on the upper metal layer of the substrate to establish electrical connections. Particularly, the top package die 14 is attached onto a substrate 12 having (in this example) two metal layers 121, 123, each patterned to provide appropriate circuitry and connected by way of vias 122. The die is conventionally attached to the upper surface of the top package substrate using an adhesive 13, typically a die attach epoxy. The die 14 and the wire bonds 16 are encapsulated with a molding compound 17 that provides protection from ambient and from mechanical stress to facilitate handling operations, and provides a surface for marking for identification. Solder balls 18 are reflowed onto bonding pads 143 on the upward-facing metal layer of the folded-over bottom package substrate to provide z-interconnection between the top and the bottom packages.
An advantage of a structure as in FIG. 4 is that the folded-over substrate provides sufficient area on the upward-facing surface of the folded-over bottom package substrate to accommodate a full array of solder balls in the top package and to accommodate more complex interconnect between the two packages. It also provides for a small package footprint. A primary disadvantage of this configuration is the high cost of the substrate and the unavailability of folding technology and equipment.
A lack of folding technology and equipment makes manufacture of the 2-stack folded flexible substrate MPM configuration more complex and more costly. The two substrates must be designed such that the pads match for the interconnect balls. If one of the packages is exchanged for one in which the substrate has a different pad arrangement (different size or different design), then the substrate for the other package must be reconfigured accordingly. This leads to increased cost for manufacture of the MPM. Moreover, all the interconnects from the top to the bottom package must be routed through the folded portion of the flex substrate at one edge of the package. This increases the routing density and increases the length of the routing traces, resulting in higher inductance and lower electrical performance of the MPM.
A common feature of all these stacked package configurations is that they enable pretesting of each package, and provide for production MPMs with higher final test yields.